1) Field of the Invention
This invention relates to a waveguide-type optical device, for example, an optical switch.
2) Description of the Related Art
Waveguide-type optical devices with metal electrodes provided on a buffer layer have been proposed to date. These conventional waveguide-type optical devices will hereinafter be described with reference to some of the accompanying drawings.
FIG. 12 is a cross-sectional view showing one example of the conventional waveguide-type optical devices. The waveguide-type optical device depicted in this drawing is a typical optical switch, which is equipped with a substrate 10, wave-guiding paths 12,14 formed in the substrate 10, a buffer layer 16 provided on a surface 10a of the substrate 10, said surface 10a being on the side of the wave-guiding paths 12,14, and electrodes 18,20 provided on the buffer layer 16.
In this typical example, the substrate 10 is a Z-cut of a LiNbO.sub.3 substrate and the wave-guiding paths 12,14 have been formed by diffusing Ti into the substrate 10. The buffer layer 16 is an SiO.sub.2 layer. The electrodes 18,20 are provided on the buffer layer 16 at regions right above the wave-guiding paths 12,14, respectively.
FIG. 13 is a cross-sectional view illustrating another example of the conventional waveguide-type optical devices, in which elements of structure similar to their corresponding elements in the above-described conventional example are indicated by like reference numerals.
In this example, the substrate 10 is an X-cut of a LiNbO.sub.3 substrate. The buffer layer is divided into three buffer layers 22,24,26 to expose the wave-guiding paths 12,14. Electrodes 28,30,32 are provided above the respective buffer layers 22,24,26.
The two waveguide-type optical devices described above have the advantage that the propagation loss and scattering of light by metal electrodes can be prevented because the electrodes are provided on the buffer layer.
As a further example of such optical switches, an optical switch with an indium tin oxide (ITO) layer as a buffer layer has been proposed, for example, in Applied Physics Letters, 47, 211 (Aug. 1, 1985).
In each of the conventional waveguide-type optical devices shown in FIGS. 12 and 13, the resistance of the buffer layer is high so that, when a voltage is applied across the electrodes to control the operation, a space charge limiting current flows into the substrate from the positive electrode and carriers are also injected into the buffer layer, especially into a part of the buffer layer, said part being located adjacent the negative electrode. As a result, space charges are produced by the injected carriers and are superimposed on an electric field developed via the electrodes for operational control. This has led the problem that the drive voltage required for the waveguide-type optical device varies.
FIG. 14 diagrammatically illustrates variations of the drive voltage. This diagram shows the results of an experiment conducted on the waveguide-type optical device shown in FIG. 12. In FIG. 14, the drive voltage is plotted along the ordinate while the time elapsed after the initiation of the application of the voltage (i.e., voltage application time) is plotted along the abscissa.
As is depicted in FIG. 14, the voltage (i.e., drive voltage)--which should be applied across the electrode to make the waveguide-type optical device output light with predetermined output power from a desired output port thereof--drops with the time elapsed after the initiation of the application of the drive voltage and, beyond a certain time point, remains at substantially a constant value. The drop of the drive voltage is about 10% of the drive voltage at the time of the initiation of the application of the voltage.
When a waveguide-type optical device is put into practical use, it is necessary to maintain constant a voltage applied across electrodes of the waveguide-type optical device so that control can be simplified. However, the value of a voltage required to obtain predetermined output power (i.e., drive voltage) eventually drops in a conventional waveguide-type optical devices as described above. This has led to the problem that light cannot be outputted with desired output power from a predetermined output port if the voltage applied across the electrodes is maintained constant.
To avoid the occurrence of such problems, an ITO layer is provided as the buffer layer in the optical switch disclosed in the literature referred to above. The optical switch is, however, accompanied by another problem that the switching speed becomes slow when a high-frequency signal is inputted.